1. Field of the Invention
The present invention relates to an optical information recording/reproducing technique and, more particularly, to an optical information recording/reproducing apparatus for enlarging a magnetic domain by moving a magnetic wall using a heating light spot, and placing a reproducing light spot in the enlarged magnetic domain.
2. Related Background Art
In recent years, in optical information recording/reproducing apparatuses using optical information recording media such as magneto-optical disks, research and development on techniques of recording information at high densities and reproducing the information have been enthusiastically made. Conventionally, the size of a data mark bearing recorded information on an optical information recording medium has been limited by the diffraction limit of an optical system. Recently, however, optical information recording/reproducing methods in which the size of a data mark is not limited to the diffraction limit of an optical system have been proposed.
For example, in the method disclosed in Japanese Laid-Open Patent Application No. 6-290496, a magnetic domain is enlarged by moving a magnetic wall using a heating light spot, and a reproducing light spot is placed in the enlarged magnetic wall so as to reproduce the recorded information of a minute mark which is equal to or smaller in size than the resolution of an optical system, thereby realizing a recording medium with a high recording density.
FIG. 1 schematically shows the optical head optical system of the recording/reproducing apparatus disclosed in the above reference. As shown in FIG. 1, a heating laser is added to the optical system of a general magneto-optical disk recording/reproducing apparatus. Referring to FIG. 1, a recording/reproducing laser source 1 emits a laser beam with a wavelength of 780 nm. A heating laser source 2 emits a laser beam with a wavelength of 1.3 .mu.m. A dichroic mirror 3 is designed to transmit 100% of 780-nm light and reflect 100% of 1.3-.mu.m light. A polarizing beam splitter 4 is designed to transmit 70 to 80% of the P-polarized light of 780-nm light and 1.3-.mu.m light and reflect 100% of the S-polarized light thereof. The diameter of a 1.3-.mu.m light beam incident on an objective lens 5 is set to be smaller than the aperture of the objective lens 5 so that the NA with respect to the 1.3-.mu.m light beam is smaller than the NA with respect to 780-nm light which passes through the entire aperture portion of the objective lens 5 to be focused. A dichroic mirror 7 is placed to prevent 1.3-.mu.m light from leaking into the signal detection system. The dichroic mirror 7 is designed to transmit 100% of 780-nm light and reflect 100% of 1.3-.mu.m light. FIG. 1 also shows a magneto-optical recording medium 6.
FIGS. 2A and 2B explain the operation of the recording/reproducing apparatus having the above optical head optical system. As shown in FIG. 2A, a recording/reproducing light spot 11 and a heating light spot 12 having a larger diameter than the recording/reproducing light spot 11 are formed on a land 15 between guide grooves 14, and the recording medium 6 is moved in the direction indicated by the arrow, thereby forming a temperature distribution like the one shown in FIG. 2B on the land 15. With this operation, a desired temperature gradient like the one shown in FIG. 2B is formed in an area within the recording/reproducing light spot 11 on the moving recording medium. An isothermal line 16 is an isothermal line of a temperature Ts. The magnetic layer of this recording medium is formed by sequentially stacking first, second, and third magnetic layers on each other. On this magnetic layer, a magnetic wall 13 is formed on the boundary portion between areas having atomic spins which are opposite in direction. When the magnetic wall 13 in the first magnetic layer is at a position Xs on the medium, since the temperature of the medium has already risen to the temperature Ts near the Curie temperature of the second magnetic layer at this position, the exchange coupling between the first and third magnetic layers is broken. As a result, the magnetic wall 13 in the first magnetic layer almost instantaneously moves to an area having a higher temperature (lower magnetic wall energy density).
When the magnetic wall 13 passes under the recording/reproducing light spot 11, all the atomic spins in the first magnetic layer within the light spot 11 are aligned in the same direction. Every time the magnetic wall 13 comes to the position Xs upon movement of the medium, the magnetic wall 13 instantaneously moves under the light spot, so that all the atomic spins within the light spot are reversed in direction and aligned in one direction. As a result, the reproduced signal amplitude remains constant and maximum regardless of the distances between recorded magnetic walls (i.e., the lengths of record marks).
The polarization plane of reflected light from the recording/reproducing light spot 11 is rotated by a magneto-optical effect. The reflected light reaches the polarizing beam splitter 4 through the objective lens 5. The S-polarized light component of the light is reflected by the polarizing beam splitter 4, and the resultant light is transmitted through the dichroic mirror 7 to be guided to the signal detection system.
In the above prior art, however, when a heating light spot is to be formed by simply using a long-wavelength light source and an objective lens with a small NA, a large loss of light occurs in a direction (track crossing direction) perpendicular to the track direction (medium moving direction) of a recording medium, a shortage of the light amount of the heating light spot 12 may occur.
In the prior art, when no heating light spot is to be used, the intensity of a recording/reproducing light spot is set to about 3 mW. When a heating light spot is to be used, the intensity of a recording/reproducing light spot is set to 1 mW. In a normal case, the intensity of a reproducing light spot is set to about 1 to 1.5 mW. The intensity density of a heating light spot is normally set to be about two to three times that of a recording light spot in the normal case.
In the above prior art, the heating light spot 12 is similar in shape to the almost isotropic recording/reproducing light spot 11, and has an outer diameter about four times that of the recording/reproducing light spot 11. When, therefore, the heating light spot 12 is almost equal in light amount to the recording/reproducing light spot 11, the intensity density of the heating light spot 12 is about 1/16 that of the recording/reproducing light spot 11. To obtain an intensity density about two to three times that of the recording/reproducing light spot 11, the intensity of the heating light spot 12 needs to be about 32 to 48 times that of the recording/reproducing light spot 11. Assume that the optical efficiency of a heating light emitting optical system is about two times that of a recording/reproducing light emitting optical system. In this case, the exit power of a heating light source must be about 16 to 24 times that of a recording/reproducing light source.
In general, the exit power of a reproducing light source is about 3 to 5 mW. In the above conventional apparatus, therefore, the exit power of the heating light source must be 48 to 120 mW. The exit power of the write light source in an optical disk apparatus is generally 35 to 50 mW. Under the circumstances, the possibility of a shortage of the light amount of a heating light spot is high.
In addition, since it is difficult to manufacture such a light source by using a high-output light source, a great increase in cost may occur.